Re: 回复：ubifs:Questions About Garbage Collection

[Richard Weinberger <richard@nod.at>]

Am 23.06.2016 um 19:55 schrieb Michal Suchanek:
> On 23 June 2016 at 19:34, Richard Weinberger <richard@nod.at> wrote:
>> Am 23.06.2016 um 19:20 schrieb Michal Suchanek:
>>> Hello,
>>>
>>> On 19 June 2016 at 19:05, Richard Weinberger <richard@nod.at> wrote:
>>>> Am 19.06.2016 um 18:14 schrieb 辉少:
>>>>> Thanks for your kind reply！Do you mean that in synchronous/asynchronous mode GC is also finished in synchronous
>>>>> /asynchronous way as writting is? A more question is，are all dirty spaces collected in the same time in synchronous mode(at this point the caller will be blocked for a long time) while in asynchronous mode dirty spaces will be collected little by little in asynchronous mode when i do not  notice？Thank you！
>>>>
>>>> Hmm, I don't fully understand your first question, can you elaborate?
>>>> As for dirty space, GC will not collect all dirty space. UBIFS tells GC how much free space an operation
>>>> will take. UBIFS calls this budget. The GC will try to produce as much free space as needed.
>>>>
>>>
>>> I guess the issue here is that UBIFS appears to do this
>>>
>>>  1) write files while there is space
>>>  2) when space is 0 perform a GC run as part of write operation
>>>
>>> when writes are synchronous the GC run is timed as part of the write.
>>
>> Exactly. :-)
>>
>>> If GC was asynchronous it would not appear as part of the synchronous
>>> write operation and would not cause as much write time jitter. When
>>> mounting a filesystem synchronous having GC synchronous is likely not
>>> what the user asked for. It's part of the internal fs bookkeeping and
>>> not part of the operation the user wants to perform synchronously.
>>>
>>> For GC to run asynchronously it would have to start before the
>>> available space is 0 so writes can still happen.
>>
>> How would the GC know how much space you need in future?
>> Sure, we would run the GC every X seconds/minutes/whatever but then the flash will wear out
>> faster since GC will always run and not only when you really need space.
> 
> I would collect garbage when the unusable space is some % of free
> space or when the free space is less than a few blocks rather than
> based on time. Nonetheless, it can happen that some of the data the GC
> copied to fresh blocks gets deleted before new blocks are written.
> 
>>
>>> Is it possible to set a parameter somewhere that would make UBIFS do this?
>>
>> Do we really need this?
> 
> Waiting until the space is 0 probably hurts performance in general and
> not only in the specific case of synchronous writes. I did not
> benchmark flash writing, though.

Well, changing the semantics when GC runs is rather easy.
So feel free to do experiments. If we can do better, we'll find
a way to implement it.

Thanks,
//richard